Measurement of low-derivative surface lenses by two-laser holography with Bi12TiO20 crystals

Refractive and profilometric analyses of lenses with large radii of curvature and/or large focal distance were performed through photorefractive holography using a Bi12TiO20 crystal as the recording medium and two red diode lasers as light sources. Both lasers were properly aligned and tuned in order to provide submillimetric synthetic wavelengths providing real-time interferograms in a two-color holography experiment. The resulting contour interferogram describes the form of the wavefront after the beam traveled back and forth through the lens. The fringe quantitative evaluation was carried out through the four-stepping technique, and the resulting phase map and the branch-cut method were employed for phase unwrapping. Exact ray tracing calculation was performed in order to establish a relation between the output wavefront geometry and the lens parameters such as radii of curvature, thickness, and refractive index. By quantitatively comparing the theoretically calculated wavefront geometry with the experimental results, errors below 1% for both refractive index and focal length were obtained.